A wireless communication network typically comprises of a number of different cells. Each cell comprises a base station which has the coverage area of the cell. Different cells may have very different characteristics. Some cells are in cities, comprising buildings of different heights and other cells are in the countryside comprising open fields or trees or bushes. Some cells are relatively small and some cells are relatively large. Further, the number of users in each cell may vary dramatically depending on the type of cell and on the movement of users. Typically, some cells have “weak spots” where the radio coverage is poor. Certain cells have spots with a high concentration of users resulting in high demand on resources in the cell. In order to improve the radio coverage or to support a great number of users at a certain location, relay stations may be used. Relay stations provide increased radio coverage of a certain area within a cell, and/or increased capacity in the certain area within the cell.
An example of such a solution with a relay node to increase radio coverage or capacity is schematically illustrated in FIG. 1a. FIG. 1a illustrates a base station 100 having a coverage area illustrated by a dotted oval. Within the cell or at least overlapping with the cell is a relay node 140 having a smaller coverage area also illustrated by a smaller dotted oval. The relay node provides services to a user equipment 190 located within the cell, or coverage area, of the relay node 140.
Relaying support is added in the Release 10 (Rel-10) version of the third Generation Partnership Project (3GPP) LTE specification. The solution is a layer 3 relay, which means that all radio protocols (layers 1-3) are terminated in the relay node. User equipments connect to the relay node over standard Uu interface, meaning that backwards compatibility with Rel-8 UEs is achieved. From a user equipment perspective, the relay node looks like an ordinary eNodeB, eNB. The relay node has no fixed backhaul, but connects wirelessly to a donor cell using the Un interface. The donor cell is controlled by a Donor eNB (DeNB) and is based on Uu protocols, with some modifications. The DeNB also serves user equipments connected directly to it.
One challenge for relays is to overcome different interference issues. One type of interference is self-interference, where a transmitted signal from a relay node interferes with a received signal from the DeNB, see FIG. 1b. In FIG. 1b, it is illustrated that in case a relay station 140 transmits a signal to a user equipment 190 simultaneously as it receives a signal from the base station 100, the two signals interfere with each other. The result may be that data comprised in the signal received from the base station 100 in the relay node 140 will be so greatly interfered with that the data is lost for the relay node 140.
Two different types of relaying are defined; outband and inband relaying. With outband relaying, the Uu and Un operates on different frequencies allowing continuous transmission on both links without self-interference issues. With inband relaying, Uu and Un interfaces share on the same frequency, but time multiplexing is introduced to avoid self-interference issues.
Time multiplexing for inband relaying is achieved by coordinating the scheduling of Un and Uu interfaces so that they do not occur at the same time. The DeNB configures the relay node with a Un subframe configuration, which informs the relay node of which subframes that can be used for Un transmission and which can be used for Uu transmission. Correspondingly, the relay node configures its user equipments with a Rel-8 defined Multi-Media Broadcast over Single Frequency Network (MBSFN) configuration to ensure that user equipments do not expect reference symbols transmitted by the relay node on Un subframes.
In LTE, specific MBSFN subframes are introduced to enable multi-cast transmission to the UEs being configured to receive such a service. In Rel-8/9, the UEs that are not configured to receive multi-cast transmission, receive only Physical Downlink Control Channel (PDCCH) in the beginning of the MBSFN subframe. PDCCH can carry an uplink grant for a future subframe, for example. In LTE Rel-10, a new transmission mode, Mode 9, is introduced. In this transmission mode, the UEs can receive also on Physical Downlink Channel (PDSCH) in a MBSFN subframe.
Since the three first time slots of MBSFN subframes can be used to transmit PDCCH from RN to UEs, it will not be possible for the relay node to simultaneously receive PDCCH from DeNB without significant self-interference. Therefore, 3GPP has specified a new R-PDCCH in Rel-10, which is used to carry downlink scheduling assignments and uplink grants from the DeNB to the relay node. In the time domain, R-PDCCH transmission starts in symbol #3 to avoid time with PDCCH transmissions from the relay node in symbols #0, #1 and #2. The frequency domain of R-PDCCH is semi-statically configured via Radio Resource Control (RRC), so that the relay node knows in advance in which frequency domain the R-PDCCH will be transmitted.
The solution for achieving inband operation through Un configuration has some characteristics when it comes to the sharing the radio resources between the two links.
In particular, if it has to be guaranteed that no self-interference is caused from the Uu link to the Un link, then no transmission may take place on the Uu link if there is a possibility that there might be data on the Un link.
The Rel-8/9 MBSFN configuration defines different ratios of Un/Un resource split that can be supported. Out of the 10 subframes of a radio frame, subframes 1, 2, 3, 6, 7, 8 in FDD and subframes 2, 3, 4, 7, 8, 9 in TDD can be configured for MBSFN, see FIG. 7. This means that at most 60% of the resources can be assigned for Un transmission. When configuring 6 backhaul subframes, the Uu link can use 40% of the resources.
The above characteristics mean that the sharing of resources between Un and Uu follow a certain granularity and limitations. In case of temporal change in traffic load between the relay node and user equipments being served by the relay node, the relay node may not be able to support the increase in traffic load. This may lead to discarding of traffic resulting in users not being able to use their user equipments as desired. It may also cause loss of revenue to the operator of the wireless network.